Mortars and concretes are brittle materials comprising a hydratable cement binder and, in the case of mortar, a fine aggregate such as sand, and, in the case of concrete, a coarse aggregate such as crushed gravel or small stones. If a structure made from mortar or concrete is subjected to stresses that exceed its maximum tensile strength, then cracks can be initiated and propagated in that structure.
The ability of the structure to resist crack initiation is understood in terms of its “strength,” which is proportional to the maximum load sustainable by the structure without cracking. This is measured by assessing the minimum stress load (e.g., the “critical stress intensity factor”) required to initiate cracking.
On the other hand, the ability of the structure to resist propagation (or widening) of an existing crack is described as “fracture toughness.” Such a property is determined by simultaneously measuring the load required to deform or “deflect” a fiber-reinforced concrete (FRC) beam specimen at an opened crack, and also by measuring the extent of deflection. The fracture toughness is therefore determined by dividing the area under a load deflection curve (generated from plotting the load against deflection of the FRC specimen) by its cross-sectional area.
Reinforcing fibers designed to increase both strength and fracture toughness properties are known and discussed in U.S. Pat. Nos. 6,197,423; 6,503,625; 6,265,056; 6,592,790; 6,596,210; and 6,773,646, which are owned by the common assignee hereof. In these patents, Rieder et al. disclosed “micro-diastrophic” polymer fibers having irregular and random displacements of polymer material, stress fractures, and microscopic elevated ridges.
Subsequently, in U.S. Pat. Nos. 6,529,525; 6,569,526; 6,758,897; and 6,863,969, also owned by the common assignee hereof, Rieder et al. disclosed polymer fibers having improved strength and fracture toughness while retaining dispersibility. By extruding and slitting a flat polypropylene film, and stretching the slit fibers using an extremely high stretch rate, Rieder et al. achieved fibers having a modulus of up to 20 Gigapascals. By avoiding the stress-fracturing of the aforementioned micro-diastrophic flattening technique, the structural integrity of the fibers could be preserved.
Slit polypropylene reinforcing fibers which provide high strength and fracture toughness in concrete are commercially available from Grace Construction Products, Cambridge, Mass., under the trade name “STRUX®.”
One of the objectives of the present invention is to employ fibers having a modulus, at least 5 Gigapascals, and more preferably at least 20 Gigapascals and more, for increasing the ability of a fiber-reinforced matrix material to resist crack initiation.
Another objective is to provide fibers that enhance the fracture toughness of the matrix material, its ability to resist deflection or widening of existing cracks. The present inventors must, in other words, now consider how best to control pull-out resistance of high modulus fiber materials. This property must be considered with respect to the situation wherein the fibers span across a crack or opening in the matrix material.
In U.S Pat. No. 4,297,414, Matsumoto disclosed polyethylene fibers having protrusions. These are made by mixing polyethylene having a melt index of not more than 0.01 at 190° C. under a load of 2.16 kg with polyethylene having a melt index of more than 0.01, thereby to obtain a mixture having a melt index of 0.01 to 10. This mixture was extruded under such conditions as to create a jagged surface, which was then stretched to generate the surface protrusions. In order to achieve this extreme melt-fracturing of the surface after the stretching treatment, it was important that “the convexities and concavities of the extruded product should be as sharp and deep as possible” (Col. 3, 11. 35-39). While no doubt making it more difficult to pull the fibers out of concrete, these distressed protrusions and concavities are believed by the present inventors to create potential breakage points or distressed features, which could lead to premature breakage of the fibers and lowering the reinforcing efficiency for a certain dosage.
One of the concerns in steel fiber product design has been to increase fiber “pull out” resistance, because this increases the ability of the fiber to defeat crack propagation. In this regard, U.S. Pat. No. 3,953,953 of Marsden disclosed fibers having “J”-shaped ends for resisting pull-out from concrete. However, such morphology can create entanglement problems and make the fibers difficult to handle and to disperse uniformly within a wet concrete mix. Also, the J-shaped ends are believed by the present inventors to cause premature breakage at the stress-point caused by the folds of the “J” shape. At column 1, lines 54-56, Marsden indicated that the end portion of his fibers are supposed to be larger in cross-section than the smallest cross-section of the shank of the filament or fiber. He preferred that the end portions of his fibers be larger in both the longitudinal and transverse planar cross-sections. (See e.g., Col. 1, lines 54-56).
A similar large-end approach was taught in Japanese Patent Application No. JP06263512A2 of Kajima, who disclosed reinforcing short fibers that gradually tapered from both ends toward the central part of the fibers. The geometry of the Kajima's fibers, which resemble two slender conical sections joined at their tops, was designed to allow tensile stress on the fiber to be dispersed into a concrete or synthetic matrix, such that the short fiber is mutually compressed and restricted, thereby resisting crack openings in the matrix. The intent of Kajima is to distribute load on the fiber to the matrix such that the load is not concentrated on one point, so that propagation of cracking in the matrix is prevented by distributing the force throughout the matrix.
The present inventors believe that prior art fibers, such as those disclosed in Marsden and Kajima, lose reinforcing efficiency, because such fibers will tend to break at a narrowed mid-section. In other words, the small waist or smallest diameter will define the maximum load-carrying capacity of the individual fiber.
It follows that at the larger ends of such fibers, an excess of material in the circumferential diameter provides anchoring of the fiber by radial compression of the fiber ends during a crack-opening event in the surrounding matrix. However, this excess fiber material at both ends does not contribute to the maximum load-carrying capacity of the fiber, due to the fact that the breakage is designed to occur at the smallest diameter.
The reinforcing performance of the fiber is not, therefore, proportional to the amount of material used in the fiber.
Kajima's tapered fibers would also be difficult to manufacture. Kajima does not describe how one is to manufacture the bi-conical shape, or how such a tapered geometry can be manufactured on a continuous basis at high speed. While it can be surmised that the tapered conical shape can be made by casting metal or polymer material in a mold, it is doubtful that such a process would be practical for high volume applications such as for reinforcing concrete. If the Kajima fibers were to be manufactured by altering conditions of extrusion, such as narrowing the die opening or stretching the extruded polymeric material to decrease the circular diameter, the surface fracturing sought by Matsumoto might occur; this would defeat the purpose of Kajima, which is to distribute forces along the body of the fiber. “Crimped” polymer fibers are known for increasing pull-out resistance from concrete and other matrix materials. For example, a sinusoidal fiber is disclosed in U.S. Pat. No. 5,981,630 of Banthia et al. and illustrated as a waveform having a profile amplitude. One problem of crimped fibers, as noted in U.S. Pat. No. 5,985,449 of Dill (Specialty Filaments), is that fiber balling (e.g., agglomeration) in concrete is difficult to avoid. Dill thus taught a bundling technique for aligning the fibers with each other so as to minimize self-entanglement.
Aside from the difficulty in dispersal, it is believed that crimping does not provide a wholly satisfactory solution to enhancing pull out resistance of the fibers. This is because a crack does not always occur at the longitudinal mid-section of a given fiber. The result is that crimped fibers can be pulled out of concrete or other matrix in something of a “snake-like” fashion.
A novel improved tapered fiber is needed which avoids the foregoing disadvantages, and which can be manufactured both conveniently and economically to achieve high reinforcing efficiency as well as controlled pull-out resistance.